Reflecting mirrors can have different shapes in order to achieve different focal lengths, and some mirrors have deformable shapes to achieve a variable focal length.
German Patent No. DE 100 01 900 A1 discloses a deformable mirror with a mirror disk and an actuator that engages concentrically on the rear side of the mirror surface. The actuator acts on a ring that is disposed behind the mirror disk and is preferably formed integrally with the mirror disk. The sphere of the mirror surface can be influenced by the cross-sectional geometrical shape of the ring and also by a centrally symmetrical weakening of the mirror disk and by static overpressure of a fluid-filled chamber behind the mirror disk.
According to the technical teaching of German Patent No. DE 39 00 467 A1, the rear side of a mirror can have a cavity and this cavity is pressurized with a larger or smaller pressure to change the geometrical shape of the mirror thereby.
In a deformable mirror according to the teaching of German Patent No. DE 100 52 249 A1, the deformable mirror element is coolable by a medium when used as a laser beam guiding component with a housing with associated mirror element.
Spherical optical elements, such as lenses or mirrors, can only focus beams close to the optical axis with a small imaging error because increasing axial separation increases imaging errors due to spherical aberration. Moreover, increasing separation and enlargement of the angle to the optical axis produces astigmatism.
One known possibility to reduce these imaging errors is to use an additional deflection mirror. As shown in FIG. 5, a generated laser beam 17 can reflect off of a circular reflecting surface of a mirror 18 that can be deformed by pressurization. An additional deflection mirror 19 is required to guide the laser beam 17 at a small angle to the adaptive mirror 18. Two mirrors are required to obtain adjustable focusing of the laser beam in the processing head of the laser-processing machine.
An imaging mirror requires aspherical deformations of the reflecting surface of the mirror for large beam deflecting angles (from the optical axis), in particular 90° deflections, in contrast to spherical mirrors, wherein the aspherical deformations can be generated (e.g., by adaptive mirrors having an elliptical or oval reflecting surface which can be loaded with pressure). Mirrors of this type are demanding to realize.